The present invention relates to digital audio signals, and more particularly to synchronous switching of digital audio data channels while maintaining block alignment between the switched digital audio signals at the output.
An internationally known standard for the interchange of digital audio is AES3-1992. Under this standard two channels of audio are digitized and then time multiplexed into a single serial digital data stream. This stream contains certain synchronizing symbols (X, Y), or preambles, that are used by a receiver of the stream to demultiplex and deserialize the two digital audio channels. In addition to the digitized audio samples, each channel within the stream also contains auxiliary information in the form of a channel status bit (C), a user data bit (U), a validity bit (V) and a parity bit (P). The validity and parity bits only pertain to the samples in which they are contained, but the channel status and user data bits form blocks of data which span 192 samples each. A special preamble (Z) in place of the X preamble in the AES stream indicates the start of a new 192 sample block.
A problem exists when it is necessary to switch between two AES streams. Current methods of synchronous switching of AES signals only do frame alignment on the signals, not block alignment. If the block timing at the output is interrupted, downstream equipment may experience a brief failure, possibly in the form of the receiver not being able to recover the AES signal for a short period of time. This dropout of the AES signal results in unwanted pops, clicks or muting. One method of circumventing this problem is to discard the auxiliary information, storing only the audio sample data in first-in, first-out (FIFO) buffers and switching between the FIFOs, and create new auxiliary information on the output by reformatting in an AES encoder so that the block alignment at the output never changes. However this method completely loses the original auxiliary information.
What is desired is a method of synchronous switching of digital audio data channels while maintaining block alignment between the switched digital audio signals at the output without losing the original auxiliary Information.